One of the most widely used techniques for controlling PMSMs is vector control, or its field oriented control (FOC) embodiment. This technique, which also uses the space vector modulation (SVM) technique, may be able to produce a stator magnetic flux, synchronous with the rotor flux, whose amplitude and related displacement (orientation) increase the electromagnetic torque produced versus current consumption. Improved accuracy measurement of the rotor position, and speed and phase currents of the motor may be desired.
FIG. 1 is a block diagram of a system for controlling the torque in a three-phase motor using the FOC technique. The feedback signals, Isd (direct current) and Isq (quadrature current), that the controllers ISO and ISQ (that typically are PI controllers) compare with the reference currents Isdref and Isqref, are obtained with a double transform of the phase currents of the motor Ia, Ib and Ic. These currents, measured by sensors, are first transformed from the three-phase stator reference system to a two-phase stator reference system (Clarke transform),
                                          I            α                    =                      I            a                          ⁢                                  ⁢                              I            β                    =                                                    I                a                                            3                                      +                                          2                ⁢                                                                  ⁢                                  I                  b                                                            3                                                    ⁢                                  ⁢                  0          =                                    I              a                        +                          I              b                        +                          I              c                                                          (        1        )            
To transform the currents Iα, Iβ in a rotor reference system (that is a time invariant system of coordinates), a further transform may be desired (Park transform) in which the angular (electrical) position θel of the rotor is used:Isd=Iα cos(θ)+Iβ sin(θ)Isq=−Iα sin(θ)+Iβ cos(θ)  (2)
The controllers ISD and ISQ, that are the controllers of the currents Isd and Isq, generate reference voltages Vsd and Vsq as a function of the difference between the reference currents and the feedback currents. These voltages, after having been transformed with the inverse of the Park transform,Vα=Vsd cos(θ)−Vsd sin(θ)Vβ=Vsd sin(θ)+Vsd cos(θ)  (3)are supplied to the PWM generator of the inverter. Therefore, in steady state conditions, i.e., constant torque, constant rotor speed, the motor is supplied with sinusoidal voltages and currents.
When a speed control loop is put in place, the reference currents Isd* and Isq* are adjusted such to make the motor speed ωrotor track a desired value ωrotor*, otherwise the reference current are selected to produce a desired electromagnetic torque Te*. Rotor electrical angular position and speed are typically measured by sensors, for example, encoders, Hall sensors, and resolvers, or may be estimated or observed by exploiting the mathematic model of the motor and known parameters.
Depending on design and/or manufacturing processes, permanent magnets synchronous motors (PMSM) may generate a non-sinusoidal back electromotive force (BEMF), i.e. the BEMF produced has, of course, relatively the same rotation frequency of the rotor, but also has a non negligible content of harmonics of a higher order.
For this reason, because of the shape of the BEMF, a torque ripple superimposes on the electromagnetic torque produced with state-of-the-art field oriented control circuits. This torque ripple may often cause an audible noise. Therefore it may be desirable to drive a three-phase electric motor with reduced generation of acoustic noise.
Chapter 2.2.4 of H. D. Stoelting, E. Kallenbach, W. Amrhein, Handbook of Fractional-Horsepower Drives, Springer, describes a technique for deleting noise by superimposing a current ick on the operational sinusoidal current curve ik upstream with respect to the current regulation block to suppress the torque ripple. A block diagram illustrating this technique is illustrated in FIG. 2 of the present application, which corresponds to FIG. 2.85 of H. D. Stoelting, E. Kallenbach, W. Amrhein, Handbook of Fractional-Horsepower Drives, Springer. Unfortunately, in many practical cases noise cannot be sufficiently reduced and remains increasingly annoying.